Based on RF transceiver for implantable medical devices

Zarlink Semiconductor company for pacemakers, Nerve Stimulator, pump and other implantable medical devices applied a ultra low power RF transceiver chip, the data transfer rates, low power consumption, with unique wakeup circuit.

This article discusses how to use the RF transceiver for intimate communication system design.

Integrated circuit (IC) and medical device development over the past 30 years has been developed at the same time.

Circuit technology contributed to the development of increasingly complex, highly integrated and compact medical device development. At the same time, health care costs continue to rise and people living in the more affluent, more fat in the body, as well as longevity, have had to rely on the base station wireless connection of implantable medical devices of new application and treatment needs. Traditionally, implantable medical devices of communication system uses very short magnetic coupling, which requires the programmer and medical equipment to tightly coupled, usually data rate is lower than 50kbps.

In order to overcome the distance limits 402MHz ~ 405MHz medical implant communication services (MICS) band in 1999 enabled, then Europe would appear similar standards.

The band was possible with long distance (usually 2m), relatively high speed wireless link. In the human body due to signal transmission characteristics, and the band working user work-compatible (such as meteorological balloons, and other auxiliary meteorological equipment) and global availability, 402MHz ~ 405MHz band is ideal for this kind of service.

For implantable medical applications of the electronic system of the enormous difficulty of low power design.

For example, the vast majority of implantable pacemaker life require longer than seven years, the maximum leakage current in 10uA ~ 20uA magnitude. Because of the need to support the pacing and on current consumption requirements, communication systems current design of the equipment within the overall average life current does not exceed the total amount of the current design, i.e. 15% 2uA ~ 3uA. Implantable medical system of transceivers must periodically "view" or monitoring external communication devices, in the view, at a very low-power state to save energy.

Design considerations

In order to be able to use MICS band, implantable medical devices requires the use of ultra low power, high performance transceiver.

Implantable device transceiver design face numerous challenges, including:

(1) 400MHz communications for low power.

Implantation of battery power is limited, and implanted battery impedance is relatively high, which limits the current from the battery slot. (2) the communication phase, for the majority of implantable devices, should be less than the current limit in 6mA. (3) is sleeping and periodic "view" to wake-up signal, at a low power. (4) the minimum of external components and physical volume. Implantable-level components of expensive, high integration can reduce costs and increase overall system reliability. (5) data transfer rate is reasonable. Currently, pacing requirements for data transfer rates greater than 20 kbps, the future design of data transfer rate is much higher. (6) systems and data transmission reliability. (7) the selectivity and interference suppression capabilities, in particular, TETRA radio standards in Europe. (8) distance typically more than two meters. The longer the distance requires sensitivity to the better, because small antenna and the effects of loss and allows distance link budget. Antenna, matching, decay and loss of changes are large, losses may be as high as $ 40dB ~ 45dB.

ZL70101 MICS transceiver in the high data transfer rate of cases with excellent low-power characteristics.

Up to the data rate 800kbps, transmit and receive current is less than 5 mA. Circuit has a unique work in 2.45 GHz ultra low-power wake-up system, an average of sleep/monitoring current less than 250nA. System integration, just need three external components (Crystal and two decoupling capacitor) and a matching network.

Medical devices can be divided into an internal non-rechargeable batteries (such as a pacemaker) class and inductive coupling power class (such as cochlear implants).

The former is strongly mining system duty potential to save power. Transceiver for most of the time is off, so off current and cyclical find communication equipment need current must particularly low (< 1-2uA). At the same time, the two situations of the transmit and receive power is low (current < 6mA).

In 2.1V ~ 3.5V power voltages, ZL70101 peak current consumption receiving/transmitting 5mA, this < include basic RF transceivers and MAC current.

MAC to ensure that users receive high integrity data, automating the most link maintenance work. In addition, the MAC Protocol provides a savings in power, transmission of a packet of timer, the timer will be implanted device receiver off for some programming good time.

To enable to joules/bits defined total power consumption minimum, meet the requirements for using the receiver sensitivity, recommended implantable transceiver using data rates as high as possible.

Need a low data rate (or even low kHz range) systems should be buffered, working at the highest possible data rate, lower duty cycle to reduce average current consumption. To send a short pulse data saves power, reduce the time window of interference. In addition, the high battery impedance system, because a pulse from the capacitor discharge shorter power on decoupling requirements may be lower.

Transceiver allows users with the receiver sensitivity, from a variety of data rate (200 kbps to 400 kbps and 800 kbps).

In order to achieve this flexibility, the system uses 2 FSK modulation FSK or 4, 200 or 400 thousand characters, variable frequency deviation (see table 1). Through the use of film, digital filtering, you can reach the lower the number ofAccording to the rate and the corresponding higher receiver sensitivity. Transceiver has a MAC bypass work mode, in which radio frequency is fully available. In this configuration, users can develop custom protocol and data transfer rates.

Overall system architecture

ZL70101 works in Implantable devices and external base station (see Figure 2).

Base station includes emission 2.45 GHz wakeup signals of additional circuits. System once adopted 2.45 GHz wakeup signal start, through 402MHz to 405MHz MICS band transceivers and Exchange data.

ZL70101 MICS chip (see Figure 3) contains the three major subsystems: a 400MHz transceiver, a wakeup receiver 2.45 GHz and a media access controller (MAC).

According to the input pin of the State determines the chips used for implantation medical equipment, or base station Programmer's transceiver.

Transceiver with an intermediate frequency (IF) low band image rejection mixer superheterodyne structure.

Low-medium frequency allows filter and modulator minimum power consumption, and high data rate, zero-if architecture related flicker noise and DC offset. FSK modulation scheme reduces emission linear amplifier, thereby reducing power consumption, and you can use the simpler by limiting the receiver. As Figure 3 won the bid to half-duplex RF transmitter with 400 MHz transmitter subsystem contains a medium frequency modulator, a mixer and a power amplifier. IF modulator will be a one (two FSK) or two (4 FSK) asynchronous digital input stream into intermediate frequency. Upconversion mixer will convert the RF frequency medium. Note that transmit and receive mode is the same as the local oscillator frequency, allowing and dead time between.

You can register custom-4.5dBm ~-17dBm (500 Ω load), the step to less than 3dB programming emission power amplifier output power.

All RF input capacitance of internal antenna matching group can fine-tune match network, for a given power settings to achieve the maximum output power, receiver noise index best. Antenna tuned to automatically scale, which takes a peak with the ADC coupled detector, take one of the calibration control state machine.

400MHz receiver subsystem MICS band signal amplification, the carrier frequency transformation to MF.

Low noise amplifier (LNA) gain to 9dB ~ 35dB programmable. On implantation medical equipment transceiver, it is recommended that a higher gain settings, and the relatively low number of gain settings can be used to select the use external LNA's base station transceiver. LNA and mixer bias current of programmability that optimization is ideal for linear (IIP3), power consumption and noise index of flexibility.

Multiple phase IF filter to reduce the frequency and adjacent channel mirror interference, and limits the noise bandwidth.

Polyphase filter after the next limiting and a receiver signal strength indicator (RSSI) module. RSSI measurement consists of a 5-bit ADC conversion, you can use industry-standard SPI interface reads. The MICS no interference channel assessment program. Note that you must first define by MICS standard without interference of a channel assessment procedures with an external instrument to determine an appropriate available channel.

To do this, also developed a highly reliable medical applications customized private agreement by MAC address, including the following major features:

(1) use Reed-Solomon forward error correction (FEC) and cycle redundancy code (CRC) error detection techniques for error correction and detect.

Suppose the original radio BER 10-3, FEC and CRC after effective BER 1.5 × 10-10. (2) fault situations block to automatically transfer, and process control to avoid buffer overflow. (3) the ability to send emergency orders and MICS high priority information. (4) be capable of processing link watchdog to ensure that the communication was not successful for 5 seconds after the broken link. (5) provide link quality diagnostic and auto-calibration control.

Ultra low-power wake up the receiver

Because storage battery energy is most important, most embedded applications are seldom MICS on the RF link.

In the extremely low power applications, most of the time, the transceiver is a current very low sleep state. In addition to send emergency command, use MICS band systems must not interfere with channel assessment program, wait for the base station to start the communication. Implantation transceiver should periodically query base station do you want to communicate.

Wake-up system is a work in 2.45GHz SRD bands of ultra low-power RF Receiver, detect and decode a special packet, the packet from the base station Launcher, and then connect the remaining power chip.

Chip can also be started directly by pin control, such as base stations start, implant device sends emergency command or using selective wake-up system implantation equipment requires this way.

A brief summary of this article

Ultra low-power wireless technology for many implantation medical equipment it is critical, including pacemakers, defibrillators, Nerve Stimulator, drug infusion system, diagnostic sensors and rapid growth of implantable Diabetes Monitor.

However, with the implanted communication systems development and support of advanced diagnostic and treatment, and wireless performance on implantation medical device battery life does not affect is critical.